Field of the Invention
The present invention is directed to a cut-off filter for integrated optics, which filter is formed as a relief-like surface grating on a film or strip waveguide and to a method of forming the filter.
What is to be understood hereinafter by the term "a cut-off filter" of this type is that it is a filter that exhibits a better sidelobe suppression when compared to a filter in the form of a relief-like surface grating having a constant corrugation depth and period on a film or strip waveguide of a uniform layer thickness.
A cut-off filter of this type is disclosed in an article by P. S. Cross and H. Kogelink, "Sidelobe Suppression in Corrugated Waveguide Filters", Optics Letter, 1 (1977), pp. 43 ff. In this filter, the improved sidelobe suppression is achieved by a variation of the corrugation depth and period of the grating in accordance with the prescribed mathematical function. Suitable functions are also specified.
Cut-off filters of this type come into consideration for integrated optical waveguide division multiplex circuits for optical communications (see page 43, first column, paragraph 1 of the above-cited article).